Bi-directional, rim generating, ducted underwater turbines for generating electrical power are known, such as the applicant's PCT application PCT/CA02/01413 to Davis et al. There is a desire to improve the flow characteristics, and therefore, the efficiency of such designs. Fixed augmentors are known to increase flow through the turbine.
U.S. Pat. No. 4,219,303 to Mouton et al. disclosed a rigid inner primary nozzle within a flexible outer nozzle which slightly overlaps the primary nozzle, creating a space between the nozzles for uni-directional flow.
U.S. Pat. No. 6,406,251 B1 to Vauthier disclosed a system of pivoting flaps on the exterior surface of a hydroturbine accepting bi-directional flow. However, this system is mechanically complicated.
The theoretical advantage of a slot in the duct of a conventional, uni-directional flow turbine with single duct was discussed in Kirke, B. Developments in Ducted Water Current Turbines, School of Engineering, Griffiths University.
It is also known to have a central space through a hydroturbine generator. U.S. Pat. No. 2,509,442 to Matheisel disclosed a ducted propeller turbine with no hub or root section to the blade which is prone to excessive deflection, vibration and high cycle fatigue, particularly as without an inner support ring the blades are cantilevered towards the centre of the unit from the housing.
U.S. patent RE38,336E (Reissue of U.S. Pat. No. 5,592,816) to Williams disclosed a hydroelectric turbine with a central open area of unrestricted flow surrounded by the blades which proposed reducing down current turbulence. U.S. Pat. No. 6,648,589 B2, also to Williams, disclosed a hydroelectric turbine with a central open area of unrestricted flow surrounded by the blades to aid in increasing the velocity of the water flowing through the single blade and to eliminate the turbulence that occurs behind the hub in traditional hub generator hydroelectric turbines. The Williams patents employ complicated hydraulic and mechanically driven generators with a uni-directional turbine blade configuration without ducting nor hydrodynamic structures to direct water flow in an efficient manner. The Williams patents do not incorporate a set of bearings at the hub to improve the structural integrity of the unit and reduce blade deflection.
The present invention satisfies the need for a structurally and mechanically simple and inexpensive to manufacture flow enhancement design which increases water flow, provides a bypass for sea life and debris thereby reducing the environmental impact of the unit, reduces vibration and hydrodynamic drag and increases the operating efficiency of underwater ducted turbines.